Hitherto known systems for calibration of pressure transducers [P. N. Modi and S. M. Seth, “Fluid Pressure and Its Measurement”, Hydraulics and Fluid Mechanics, 1392 pp. (1981)] describe a Dead-Weight Pressure Gauge wherein a cylindrical plunger slides within a vertical hollow cylinder, which is filled with oil. A pressure transducer, which is to be calibrated, is set in parallel with the dead-weight gauge. Viscous oil is slowly pumped into the cylinder (and the pressure inlet of the said pressure transducer to be calibrated). The oil, which enters the cylinder under pressure, exerts a force on the plunger, thereby lifting the plunger and balancing it against the oil pressure. This force is balanced by the weights loaded on the top of the plunger. When the oil pressure and the weights balance, the oil pressure is known from the weights and the diameter of the plunger. The pressure of the oil being thus known, the attached pressure transducer can be calibrated. The plunger can be loaded with various known weights, and the pressure transducer can be calibrated against the corresponding pressure values. The disadvantage of this arrangement is the error involved in the estimated oil-pressure due to frictional resistance offered to the motion of the plunger in the cylinder. This error can be minimized if the plunger is carefully ground, so as to fit with the least permissible clearance in the cylinder, but not completely removed. Moreover, the whole mass needs to be rotated by hand before the readings are taken. Another drawback is that the number of calibrated weights available with a dead-weight tester is limited, thereby impeding the calibration of the pressure transducer at close pressure intervals.
An alternate system, [T. G. Beckwith and N. L. Buck, “Measurement of Pressure”, Mechanical Measurements, Addison-Wesley Publishing Company, 642 pp. (1969)], describes a dead-weight tester commonly used as a source of static pressure for calibration purposes, and is basically a pressure-producing and pressure-measuring device, wherein the resulting pressure is estimated from knowledge of the applied weights and the piston area. In this device, the pressure transducer to be calibrated is connected to a chamber filled with a fluid (oil) whose pressure can be adjusted by some type of pump and bleed valve. The chamber also connects with a vertical piston-cylinder to which various standard weights may be applied. The pressure is: slowly built up until the piston and weights are seen to “float”, at which point the fluid “gage” pressure (i.e., pressure above atmosphere) must equal the deadweight supported by the piston, divided by the piston area. A drawback of this methodology is that a number of refinements and corrections are necessary to achieve highly accurate results. The frictional force between the cylinder and piston must be reduced to a minimum and/or corrected for. Another drawback is that corrections are needed for temperature effects on areas of piston and cylinder, air and pressure-medium buoyancy effects, local gravity conditions, and height differences between the lower end of the piston and the reference point for the pressure transducer being calibrated. Further, as the piston assembly itself has weight, conventional dead-weight gages are not capable of measuring pressures lower than the piston weight/area ratio.
Another system, [E. O. Doebelin, “Pressure and Sound Measurements”, Measurement Systems: Application and Design (3rd Edition, International Student Edition), pp. 404-462 (1983)], describes a U-tube manometer whose operation is based on the comparison of the unknown pressure (P) with the gravity (g), based on the well-known expression; P=hρg, where h is the height of the liquid column in the tube, and p is the density of the liquid. The cross-sectional area of the tubing (even if not uniform) has no effect. At a given location (given value of g) the sensitivity depends on only the density of the manometer fluid. An advantage of this device is that it is self-balancing and has a continuous rather than stepwise output. However, a disadvantage of this device is that it becomes unwieldy at high pressures because of the long liquid columns involved. Further, to realize the high accuracy possible with manometers, often a number of corrections need to be applied. When visual reading of the height h is employed, the engraved-scale's temperature expansion must be considered. The variation of the value of ρ of the manometer fluid with temperature must be corrected and the local value of g determined. Additional sources of error are found in the non-verticality of the tubes and the difficulty in reading h because of the meniscus formed by capillarity.
Yet another system [S. Vamagy, “Pressure Calibration Systems and Control Valve Assemblies”, U.S. Pat. No. 4,698,998 {assigned to Consolidated Controls Corporation} describes a pressure calibration system for calibrating high-pressure transducers and gaging systems. The said pressure calibration system incorporates a pressure cylinder, which is filled with compressed nitrogen gas through a fill port. The pressure cylinder is filled, from an external tank of compressed nitrogen, by pneumatically connecting them through a “pressure cable” hose and a plurality of pressure regulator valves, pressure isolation valves, vent valves, vent isolation valves, side ports, end ports, vernier balancing conduits, and knobs. A “supply pressure” gage mounted on the front-panel of the pressure calibration system indicates the pressure in the internal storage tank. A test port provided on the front panel of the calibration system provides a means for connection of the pressure equipment that is to be calibrated. The calibration of the pressure transducer/gauge is performed against a self-calibrating pressure transducer. An advantage of the system of this invention is that it is self-contained and portable, and incorporate control valves and isolation valves, thereby providing positive shutoff, combined with extremely low torque operation, permitting fine manual adjustment of calibration pressure. Another advantage of this system is that it is capable of providing pressure calibration over a working range extending from a vacuum to 10,000 psi. A disadvantage of this system, however, is that its calibration accuracy is limited by the temnperature-sensitivity of the pressure-standard against which the pressure calibration is performed. Another limitation of this system is that it does not provide a means to study the temperature-dependence of the pressure transducers being calibrated. Yet another disadvantage of this system is that the number of pressure transducers that can be calibrated simultaneously is limited to two.